Lagrangian Point -- In Lagrangian mechanics, a Lagrangian point (or L-point) is one of five positions in space where the gravitational fields of two bodies of substantial but differing mass combine to form a point at which a third body of negligible mass would be stationary relative to the two bodies.
Bodies at the L-point will not move relative to the parent bodies if they are not perturbed by other gravitational forces. They are sometimes also referred to as libration points.
The...

Saturn's moon Phoebe -- Phoebe is the outermost of Saturn's known moons. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus). It was discovered by William Henry Pickering in 1898.
Most of Saturn's moons have very bright surfaces, but Phoebe's albedo is very low (.06), as dark as lampblack. All of Saturn's moons except for Phoebe and Iapetus orbit very nearly in the plane of Saturn's equator.
Phoebe's orbit is retrograde, inclined almost 175, and is...

Saturn's moon Hyperion -- Hyperion is a moon of Saturn discovered by Bond and Lassell in 1848. It is the largest highly irregular (non-spherical) body in the solar system (Proteus is quite a bit larger but is almost spherical).
It seems likely that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past. The largest crater on its surface is approximately 120 kilometers in diameter and 10 kilometers deep.
Like most of Saturn's moons, Hyperion's low...

Saturn's moon Titan -- Titan is the planet Saturn's largest moon. It is larger than either of the planets Mercury or Pluto and is the second-largest moon in the solar system after Ganymede (it was originally thought to be slightly larger than Ganymede, but recent observations have shown that its thick atmosphere caused overestimation of its diameter).
Titan was discovered on March 25, 1655 by the Dutch astronomer Christian Huygens, making it one of the first non-terrestrial moons to be...

Saturn's moon Rhea -- Rhea is the second largest moon of Saturn. It was discovered in 1672 by Giovanni Cassini.
Rhea is an icy body with a density of about 1.24 gm/cm3. This low density indicates that it has a rocky core taking up less than one-third of the moon's mass with the rest composed of water-ice.
Rhea's features resemble those of Dione, with dissimilar leading and trailing hemispheres, suggesting similar composition and histories. The temperature on Rhea is -174°C in direct...